Secondary batteries are widely used in a household stationary electrical storage device, an electric vehicle power source, a personal computer, a satellite, and the like. However, secondary batteries are known to be prone to solution leakage due to overcharging and failure due to overdischarging, and thus required to be operated based on appropriate recognition of the state of charge or depth of discharge. For primary batteries, too, operation based on recognition of the depth of discharge is effective in, for example, determination of a replacement timing.
Typically, the state of charge of a battery has been managed by measuring the voltage of the battery in an energized or opened state (Patent Literature 1). This is because the voltage of a conventional battery tends to gradually decrease with discharging and thus it is possible to recognize the state of discharge from a reached voltage. For some batteries, however, it is difficult to estimate the state of discharge only from voltage.
Recent progress of battery development has been accelerating the tendency to request high safety. Known examples of batteries particularly focused on safety include a battery in which ionic liquid is used as electrolytic solution and a battery including an electrode made of a material with limited oxygen generation. These batteries are also needed to be operated based on appropriate recognition of the state of charge or depth of discharge to avoid solution leakage due to overcharging and failure due to overdischarging.
The operation of a battery also requires appropriate recognition of the health of the battery in addition to the state of charge or depth of discharge. Operation of a battery having a degraded health potentially results in serious accidents such as firing and explosion.